Chadwick managed to work out the mass of the neutron from his experiments and by 1934 it was quite clear that the neutron was more massive than the proton. The best modern data have the mass of the proton set at 1.007825, and that of the neutron just a trifle greater at 1.008665. The fact that the neutron was just about as massive as the proton was to be expected if the neutron were a proton-electron combination. It was also not surprising that the isolated neutron eventually breaks up, giving up an electron and becoming a proton. Out of any large number of neutrons, half have turned into protons in about 12 minutes. Nevertheless, although in some ways we can explain the neutron by speaking of it as though it were a proton-electron combination, it really is not. A neutron has a spin of ½ while a proton-electron combination would have a spin of either 0 or 1. The neutron, therefore, must be treated as a single uncharged particle. The Proton-Neutron Theory As soon as the neutron was discovered, the German physicist Werner Karl Heisenberg (1901- ) revived the notion that the nucleus must be made up of protons and neutrons, rather than protons and electrons. It was very easy to switch from the latter theory to the former, if one simply remembered to pair the electrons thought to be in the nucleus with protons and give the name neutrons to these combinations. Thus, the helium-4 nucleus, rather than being made up of 4 protons and 2 electrons, was made up of 2 protons and 2 proton-electron combinations; or 2 protons and 2 neutrons. In the same way the oxygen-16 nucleus instead of being made up of 16 protons and 8 electrons, would be made up of 8 protons and 8 neutrons. The proton-neutron theory would account for mass numbers and atomic numbers perfectly well. If a nucleus was made up of _x_ protons and _y_ neutrons, then the atomic number was equal to _x_ and the mass number to _x_ + _y_. (It is now possible to define the mass number of a nucleus in modern terms. It is the number of protons plus neutrons in the nucleus.) [Illustration: _Werner Heisenberg_] The proton-neutron theory of nuclear structure could account for isotopes perfectly well, too. Consider the 3 oxygen isotopes, oxygen-16, oxygen-17, and oxygen-18. The first would have a nucleus made up of 8 protons and 8 neutrons; the second, one of 8 protons and 9 neutrons; and the third, one of 8 protons and 10 neutrons. In each case the atomic number is 8. The mass numbers however would be 16, 17, and 18, respectively. In the same way uranium-238 would have a nucleus built of 92 protons and 146 neutrons, while uranium-235 would have one of 92 protons and 143 neutrons.